Such a plasma electrode with inserted cooling tube has become known, for example, with the subject matter of EP 2 082 622 B1. Reference is hereby made to that disclosure and the mode of operation of a plasma arc torch. It shall be deemed to be incorporated in its entirety in the disclosure of the present invention.
In the operation of plasma arc torches, the problem exists that the electrode core disposed on the front side of the electrode body in an electrode core holder is exposed to working temperatures reaching up to 1500° C. For this reason, the electrode body of the plasma electrode needs to be adequately cooled. This is accomplished according to the subject matter of EP 2 082 622 B1 in such a way that the plasma electrode in the form of a hollow cylindrical electrode body has inserted therein a likewise hollow cylindrical cooling tube, through which a coolant flow passes in the supply and return flow. The coolant flow is routed through the central internal bore of the cooling tube to the front in the direction of the electrode core holder of the electrode body, where it is deflected in the bottom end of the electrode body and flows back on the outside of the cooling tube and on the inner circumference of the electrode body.
The cooling tube is subjected to considerable thermal expansion, and care must be taken that during thermal expansion thereof it does not disrupt the coolant flow. For this purpose, EP 2 082 622 B1 proposes to provide the front face side of the cooling tube situated next to the electrode core holder with a spacer.
The spacer is formed in the bottom end of the electrode body as an insertable disk or as intersecting bars and is intended to form a stop surface for the front end of the cooling tube against the electrode body.
It is a disadvantage of this known spacer, however, that it must be inserted or press fit as a separate part into the electrode body, which is associated with increased expenditure of time and effort.
It is another disadvantage that it is not part of the cooling tube and does not take part in the longitudinal expansion of the cooling tube, which poses a risk that the front end of the cooling tube may come to rest on the spacer in a sealing manner, and the coolant flow is impaired as a result.
The invention is therefore based on the aim of improving a plasma electrode for a plasma arc torch of the type mentioned at the beginning in such a way that an improved spacing-maintaining support for the cooling tube in the interior of the hollow cylindrical electrode body is ensured.
To achieve this aim, the invention relates to a cooling tube for a plasma arc torch, comprising a hollow cylindrical electrode body having a central internal bore, at the front end of which an electrode core holder with an electrode core inserted therein is disposed, and a hollow cylindrical cooling tube inserted in a sealing manner into the internal bore, which in the internal bore thereof has a cooling channel configured as a supply passage and in the space between the outer circumference thereof and the inner circumference of the electrode body forms a cooling channel configured as a return passage, characterized in that the cooling tube on the side thereof facing toward the electrode core holder has spacing means (e.g. a spacer or a spacer disk or wires or rods) which are suitable for face-end engagement against the electrode core holder.
It is an essential feature of the invention that the cooling tube has on its inside facing towards the electrode core holder a spacer suited to engage against the electrode core holder. Accordingly, any spacing means that are suitable for a displacement-limiting engagement of the cooling tube against the electrode core holder are claimed as essential to the invention.
With the technical teaching herein a significant advantage is achieved over the prior art as shown in EP 2 082 622 B1, because, according to the invention, the spacing means is no longer part of the hollow cylindrical electrode body, but part of the cooling tube itself.
This spacer is incorporated in the form of, for example, a spacer disk in the interior of the cooling tube and therefore—because it is connected to the cooling tube in a fixed manner—takes part in the longitudinal expansion of the cooling tube. This was not possible in the prior art.
For this reason, an ever-constant flow of coolant through the cooling tube is ensured, irrespective of the rapidly changing linear expansion of the cooling tube which is directed with the front face side thereof sometimes more, sometimes less towards the bottom of the hollow cylindrical electrode body. If—as in the prior art—a fixed spacer disk is disposed in that region, this leads to an impairment of the coolant flow. This is avoided in the invention.